The present invention relates generally to micro-mechanical devices, and more particularly, to a micrometer sized vertical thermal actuator with controlled bending that is capable of repeatable and rapid movement of a micrometer-sized device off the surface of a substrate.
Fabricating complex micro-electro-mechanical systems (MEMS) and micro-optical-electro-mechanical systems (MOEMS) devices represents a significant advance in micro-mechanical device technology. Presently, micrometer-sized analogs of many macro-scale devices have been made, such as for example hinges, shutters, lenses, mirrors, switches, polarizing devices, and actuators. These devices can be fabricated, for example, using Multi-user MEMS processing (MUMPs) available from Cronos Integrated Microsystems located at Research Triangle Park, North Carolina. Applications of MEMS and MOEMS devices include, for example, data storage devices, laser scanners, printer heads, magnetic heads, micro-spectrometers, accelerometers, scanning-probe microscopes, near-field optical microscopes, optical scanners, optical modulators, micro-lenses, optical switches, and micro-robotics.
One method of forming a MEMS or MOEMS device involves patterning the device in appropriate locations on a substrate. As patterned, the device lies flat on top of the substrate. For example, the hinge plates of a hinge structure or a reflector device are both formed generally coplanar with the surface of the substrate using the MUMPs process. One challenge to making use of these devices is moving them out of the plane of the substrate.
Coupling actuators with micro-mechanical devices allows for moving these devices out of the plane of the substrate. Various types of actuators, including electrostatic, piezoelectric, thermal and magnetic have been used for this purpose.
One such actuator is described by Cowan et al. in xe2x80x9cVertical Thermal Actuator for Micro-Opto-Electro-Mechanical Systemsxe2x80x9d, v.3226, SPIE, pp. 137-46 (1997). The actuator 20 of Cowan et al. illustrated in FIG. 1 uses resistive heating to induce thermal expansion. The hot arm 22 is higher than the cantilever arm 24, so that thermal expansion drives the actuator tip 26 toward the surface of the substrate 28. At sufficiently high current, the downward deflection of the actuator tip 26 is stopped by contact with the substrate 28 and the hot arms 22 bow upward. Upon removal of the drive current the hot arms 22 rapidly xe2x80x9cfreezexe2x80x9d in the bowed shape and shrink, pulling the actuator tip 26 upward, as illustrated in FIG. 2.
The deformation of the hot arm 22 is permanent and the actuator tip 26 remains deflected upward without applied power, forming a backbent actuator 32. Further application of the drive current causes the backbent actuator 32 to rotate in the direction 30 toward the surface of the substrate 28. The backbent actuator 32 of FIG. 2 is typically used for setup or one-time positioning applications. The actuators described in Cowan et al. are limited in that they cannot rotate or lift hinged plates substantially more than forty-five degrees out-of-plane in a single actuation step.
Harsh et al., xe2x80x9cFlip Chip Assembly for Si-Based Rf MEMSxe2x80x9d Technical Digest of the Twelfth IEEE International Conference on Micro Electro Mechanical Systems, IEEE Microwave Theory and Techniques Society 1999, at 273-278; Harsh et al., xe2x80x9cThe Realization and Design Considerations of a Flip-Chip Integrated MEMS Tunable Capacitorxe2x80x9d 80 Sensors and Actuators 108-118 (2000); and Feng et al., xe2x80x9cMEMS-Based Variable Capacitor for Millimeter-Wave Applicationsxe2x80x9d Solid-State Sensor and Actuator Workshop, Hilton Head Island, S.C. 2000, at 255-258 disclose various vertical actuators based upon a flip-chip design. During the normal release etching step, the base oxide layer is partially dissolved and the remaining MEMS components are released. A ceramic substrate is then bonded to the exposed surface of the MEMS device and the base polysilicon layer is removed by completing the etch of the base oxide layer (i.e., a flip chip process). The resultant device, which is completely free of the polysilicon substrate, is a capacitor in which the top plate of the capacitor is controllably moved in a downward fashion toward an opposing plate on the ceramic substrate. The device is removed from the polysilicon substrate because stray capacitance effects of a polysilicon layer would at a minimum interfere with the operation of the device.
Lift angles substantially greater than forty-five degrees are achievable with a dual-stage actuator system. A dual-stage actuator system typically consists of a vertical actuator and a motor. The vertical actuator lifts the hinged micro-mechanical device off of the substrate to a maximum angle not substantially greater than forty-five degrees. The motor, which has a drive arm connected to a lift arm of the micro-mechanical device, completes the lift. One such dual-stage assembly system is disclosed by Reid et al. in xe2x80x9cAutomated Assembly of Flip-Up Micromirrorsxe2x80x9d, Transducers xe2x80x297, Int""l Conf. Solid-State Sensors and Actuators, pp. 347-50 (1997). These dual stage actuators are typically used for setup or one-time positioning applications.
The dual-stage actuator systems are complex, decreasing reliability and increasing the cost of chips containing MEMS and MOEMS devices. As such, there is a need for a micrometer sized vertical thermal actuator with controlled bending that is capable of repeatable and rapid movement of a micrometer-sized device off the surface of the substrate.
The present invention is directed to a micrometer sized vertical thermal actuator with controlled bending capable of repeatable and rapid movement of a micrometer-sized optical device off the surface of the substrate. Controlled bending maximizes the displacement of the present vertical thermal actuator.
The vertical thermal actuator is constructed on a surface of a substrate. At least one hot arm has a first end anchored to the surface and a free end located above the surface. A cold arm has a first end anchored to the surface and a free end. The cold arm is located above the hot arm relative to the surface. A flexure is formed in the cold arm near the first end thereof adapted to provide controlled bending. A member mechanically and electrically couples the free ends of the hot and cold arms such that the actuator bends generally at the flexure so that the member moves away from the substrate when current is applied to the at least the hot arm.
The flexure comprises at least one of a recess, depression, cut-out, hole, location of narrowed, thinned or weakened material, alternate material or other structural features or material change that decreases resistance to bending in that location. In one embodiment, the hot arm and the cold arm comprise a circuit through which electric current is passed. In another embodiment, a grounding tab electrically couples the hot arm to the substrate. In the embodiment with the grounding tab, the cold arm can optionally be electrically isolated from the hot arm.
In one embodiment, a reinforcing member is formed in the cold arm. The reinforcing member typically extends from proximate the flexure to proximate the free end thereof. The reinforcing member can be integrally formed in the cold arm. In one embodiment, the reinforcing member extends longitudinally along the cold arm, such as one or more ridges extending longitudinally along the cold arm.
In one embodiment, the reinforcing member is located directly above the hot arm. The cold arm can be located directly over the hot arm. The first end of the hot arm can be attached to the substrate adjacent to the first end of the cold arm or offset from the first end of the cold arm. A metal layer optionally extends along the cold arm. In one embodiment, the at least one hot arm comprises two hot arms each having a first end anchored to the surface and free ends located above the surface.
In another embodiment, the vertical thermal actuator includes at least one hot arm with a first end anchored to the surface and a free end located above the surface. A cold arm has a first end anchored to the surface and a free end. The cold arm is located above the hot arm relative to the surface. A reinforcing member is formed in a first portion of the cold arm. A second portion of the cold beam without the reinforcing member is adapted to provide controlled bending of the vertical thermal actuator. A member mechanically and electrically couples the free ends of the hot and cold arms such that the member moves away from the substrate when current is applied to at least the hot arm.
In another embodiment, the vertical thermal actuator has a first beam with a first end anchored to the surface and a free end located above the surface. A second beam has a first end anchored to the surface and a free end located above the surface. A member electrically and mechanically couples the free end of the first beam to the free end of the second beam. A third beam has a first end anchored to the surface and a free end mechanically coupled to the member. The third beam is located above the first and second beams relative to the surface. A flexure is formed in the third beam near the first end thereof adapted to provide controlled bending. First and second electrical contacts are electrically coupled to the first ends of the first and second beams, respectively, such that current supplied to the first and second contacts causes the first and second beams to thermally expand and the member to move in an arc away from the substrate.
In one embodiment, the third beam is located generally over the first and second beams. The third beam may optionally include a metal layer. The first and second beams are generally parallel to the first surface in an unactivated configuration. Electric current is applied to the first and second electric contacts in an activated configuration so that the first and second beams curved upward away from the surface of the substrate.
In one embodiment, the first end of the third beam is electrically isolated from the substrate. In another embodiment, at least a portion of the current in the first and second beams passes through the third beam. The first and second beams can optionally be electrically coupled to the substrate by a grounding tab.
In another embodiment, the vertical thermal actuator has a first beam with a first end anchored to the surface and a free end located above the surface. A second beam has a first end anchored to the surface and a free end located above the surface. A member electrically and mechanically couples the free end of the first beam to the free end of the second beam. A third beam has a first end anchored to the surface and a free end mechanically coupled to the member. The third beam is located above the first and second beams relative to the surface. A reinforcing member is formed along a first portion of the third beam. A second portion of the third beam without the reinforcing member is adapted to provide controlled bending of the vertical thermal actuator. First and second electrical contacts are electrically coupled to the first ends of the first and second beams, respectively, such that current supplied to the first and second contacts causes the first and second beams to thermally expand and the member to move in an arc away from the substrate.
A plurality of vertical thermal actuators can be formed on a single substrate. At least one optical device can be mechanically coupled to the vertical thermal actuator. The optical device comprises one of a reflector, a lens, a polarizer, a wave-guide, a shutter, or an occluding structure. The optical device can be part of an optical communication system.